Various processes are now known for growing crystalline bodies. One process which has been devised for growing crystalline bodies with diverse cross-sectional shapes with excellent dimensional control over long lengths is the so-called EFG technique which is exemplified and described in varying detail in U.S. Pat. Nos. 3,591,348, 3,687,633 and 3,953,174. In the EFG method, a wettable capillary die conducts the melt from a reservoir supply (usually contained in a crucible) to the growing crystal interface just above the top of the die, with the shape of the resulting crystalline body being determined by the shape of the growth meniscus which in turn is controlled by the perimeter of the die top. Another process for growing crystalline bodies with controlled cross-sectional shapes is disclosed by U.S. Pat. No. 4,000,030, issued Dec. 28, 1976 to T. F. Ciszek. In the latter patent the method described involves use of a submerged projection extending above the level of the melt with the crystal growth occurring from a melt meniscus formed over the upper end of the projection.
The foregoing methods have been applied to or considered for growing silicon for use in fabricating photovoltaic solar cells. The growth of silicon for use in making solar cells is complicated by the fact that the presence of crystallographic defects and certain impurities in the silicon have an adverse effect on solar cell efficiency. The presence of carbon and oxygen impurities in EFG silicon has been noted.
As in the Czochralski or dendritic web melt-growth systems for silicon, carbon and oxygen levels in EFG grown silicon ribbon can be expected to be influenced both by the type of crucible used and the composition of the ambient gases in contact with the melt. However, important distinctions must be made for the EFG process which limit the parallel that may be drawn in comparing the processes by which carbon and oxygen reach steady-state concentrations in the crystalline product. This situation arises in part from the isolation of the crucible (bulk) melt from the melt ahead of the growth interface (bounded by the meniscus at the EFG die top) dictated by the geometric configuration of the die, and in part because of the relatively high EFG growth speeds. The separation of the bulk and the meniscus allows temperature differences as large as 50 degrees C. to 100 degrees C. to be maintained between them under typical growth conditions, with the result that the mechanism controlling the occurrence of oxygen and silicon in EFG-grown crystals is not exactly the same as in the Czochralski process. The effect of carbon and oxygen impurities is not known with certainty but both negative and positive influences of these on ribbon quality have been suggested.
It has been recognized that the presence of precipitated carbon in silicon causes the leakage current to be higher without any notable change in forward characteristics (see N. Akiyama et al., Lowering of Breakdown Voltage of Semiconductor Silicon Due to the Precipitation of Impurity Carbon, Appl. Physics Lett., Vol. 22, No. 12, pp. 630-631, June 15, 1973). There has been disagreement as to whether or not the presence of oxygen in semiconductor silicon is harmful, particularly if impurity carbon also is present. It has been suggested that oxygen should be eliminated or reduced to a negligible level in order to maximize carrier lifetime. On the other hand, U.S. Pat. No. 4,040,895 suggests that a reduction in leakage currents occurs at higher oxygen levels, e.g. 13.times.10.sup.17 to 17.times.10.sup.17 atoms/cc.
Other impurities which tend to occur as solutes in silicon bodies produced by the EFG and Ciszek and methods, and which have been found to have an adverse effect on the electronic properties of silicon, are: iron, titanium, copper, zirconium, molybdenum, aluminum, manganese and copper. Silicon carbide also occurs as an inclusion in the product. These additional impurities, like carbon and oxygen, may be derived from the dies, crucibles, associated heat control members such as heaters, heat shields and insulators, and other furnace components and the ambient environment in the furnace. These additional impurities tend to be distributed throughout a silicon ribbon so as to reduce carrier lifetime generally through the ribbon and thus limit the conversion efficiency of solar cells made therefrom and also the total yield of high efficiency solar cells. As a consequence, the preferred practice in growing silicon ribbon by EFG has been to (a) make the dies, crucibles and furnace components out of mateials with as high a purity as possible and (b) carry out the growth process in an inert gaseous environment using a gas of as high purity as possible.
The coice of die and crucible material is complicated by the fact that molten silicon reacts with and/or dissolves most substances that may be likely candidates as die or crucible materials. Since a degree of reactivity between molten silicon and the die is unavoidable, it is desirable that the reaction product be electrically neutral in the silicon crystal or, if insoluble in silicon, be structurally compatible in order not to generate an excessive density of crystallographic defects which would lead to excessive polycrystallinity. Additionally the die must be arranged and made of a material such that a crystallization front of suitable configuration may be maintained at all times, thereby to reduce the occurrence of discoloration defects in the crystals (in this connection it is to be noted that in the usual case a silicon ribbon grown by EFG is not an ideal single crystal but instead is generally rather imperfect in nature). In the growth of silicon, fused quartz, silicon nitride, silicon carbide and graphite have been considered most seriously as possible die materials. Fused quartz has been rejected since it has barely wetted by liquid silicon; silicon nitride is unacceptable since it tends to react too rapidly with molten silicon; silicon carbide is wetted by silicon and has adequate strength at the melting point of silicon, but the difficulty of machining silicon carbide per se makes it unacceptable in the case of capillary dies for growing relatively thin ribbons, such as ribbon with a thickness of 0.006 to 0.0020 inch. Also silicon carbide in forms suitable for making capillary dies is not available in adequate purity.
Because of the limitations of the foregoing die materials, current EFG technology is based upon graphite dies since graphite has adequate strength at the melting point of silicon, is easily machineable, is available commercially in forms suitable for making capillary dies in greater purity than silicon carbide, and is wetted adequately and in a stable manner by silicon (in current EFG technology, it is preferred that the crucibles also be made of graphite). However, the use of graphite dies is limited by the tendency for silicon carbide crystals to form at the die top as a consequence of the reaction of graphite and silicon (frequently to the point of stopping ribbon growth or providing variations in the shape of the ribbon or causing crystallization defects in the form of grain boundaries, voids or dislocations). These particles disturb the crystallization front and also tend to be picked up as occlusions by the growing crystal. With regard to silicon carbide occlusions, it is well established that silicon ribbons grown by EFG using graphite dies can have silicon carbide particles at levels which are harmful to solar cell performance and that a reduction in the occurrence of silicon carbide particles in the ribbon tends to result in an improvement in the yield of 10-12% efficiency solar cells obtainable from such ribbons.
The typical inert gas used in order to reduce the occurrence of impurities in the grown crystal is argon, although other insert gases also have been used or suggested. In any event, the usual procedure is to use inert gases which are substantially free of other gases, i.e., contain less than 5 ppm of any other gas with the exception of oxygen and water, the latter each being present in quantities as high as 10-25 ppm. The inert gas is usually caused to flow through the furnace at a controlled rate calculated not to disturb the crystallization front while assuring that any volatile impurities in the region of the growth zone will be swept out of the furnace so as to reduce the likelihood of the same being picked up by the growing crystalline body.
Notwithstanding the careful control of the composition of the die, crucible and other furnace components and the purity and rate of flow of the inert gas in the region of the growth interface, unpredicted variations in silicon ribbon quality have continued to be observed. Some of the variations appear to be due to the occurrence of large silicon carbide particles on the ribbon surface or in the ribbon at its surface, and/or the presence of high levels of carbon in the ribbon. Also in practical EFG ribbon growth systems it is difficult to control the oxygen content in the furnace in a reproducible manner and at low concentrations. Therefore, the concentration and effect of oxygen in the ribbon tends to vary unpredictably.
Accordingly, the primary object of this invention is to provide a method of growing crystalline silicon bodies using a wettable capillary die or equivalent so as to substantially reduce the formation of large silicon carbide particles at the liquid/solid interface, reduce the occurrence of silicon carbide in the grown crystalline body, and improve the electronic quality of the product. A more specific object of the invention is to provide a method of growing crystals of silicon and the like using the EFG or Ciszek methods so as to substantially improve the solar energy conversion efficiency of solar cells made from such crystals.
The foregoing and other objects hereinafter described or rendered obvious are achieved by deliberately modifying the ambient atmosphere in the crystal growing furnace, particularly at or in the region of the growth interface. More specifically, it has been discovered that silicon ribbon suitable for the manufacture of relatively high efficiency solar cells can be produced on a more consistent basis using a graphite capillary die and a graphite crucible if the ambient atmosphere is made to include suitable amounts of one or more carbon-containing gases, so as to modify the growth conditions to such an extent that significant improvements in ribbon properties result. Specifically and preferably the ambient atmosphere comprises a gas or gases containing both carbon and oxygen such as carbon monoxide (CO) or carbon dioxide (CO.sub.2). Alternatively the ambient atmosphere may be made to include a hydrocarbon such as methane (CH.sub.4) .